The digital subscriber loop interface circuit usually will comprise two parts; a transmitter and a receiver. The function of the transmitter is to put a series of pulses, usually shaped by some form of filter, on the loop. In the U-interface of an ISDN (Integrated Services Digital Network) system, these pulses are likely to be 4-level pulses, particularly encoded as the so-called 2B1Q code (two binary, one quaternary) recommended by the American National Standards Institute T1 Working Group.
The function of the receiver is to detect pulses being sent from the far end of the loop, which is difficult because these pulses are distorted. One source of distortion is coupling of the transmit pulses, being put onto the loop, directly across the hybrid circuit and into the receiver input as "echoes", which is a common problem when operating on a two-wire system.
Such transmit pulse echoes are removed by echo-cancellation, typically using a transversal filter to derive a function of the transmit signal for subtraction from the received signal.
Basically a code such as Alternate Mark Inversion (AMI), which guarantees that alternate pulses will be inverted, does not generate long tails. The echo tail is generated by a pulse of a certain amplitude and proceeds to decay through the succeeding bit periods or time slots. Since the next pulse will be of the opposite polarity it will cancel out most of the tail of the preceding pulse. For a non-redundant code like 2B1Q, however, the succeeding pulses might be the same amplitude and the same polarity so rather than a succeeding pulse cancelling out the tail, it can in fact help to maintain the tail. As a result, the long tail presents a considerable problem for the echo canceller.
Several ways of approaching the problem are possible. One, for example, takes account of the fact that the hybrid tends to reduce the low frequency component and hence introduces the tail. To overcome this effect, however, would require that the inductance of the hybrid transformer, and hence its size, be increased. Because this is an analogue part of the circuit, the increase could not be achieved without incurring a significant cost increase at the front end.
A second way is to increase the high pass filtering at the receiver input which would have the effect of reducing the length of the tail. This is not preferred, however, because it would increase noise, particularly since such a filter is located early in the circuit where the signal is still very noisy.
A third possibility is to use more taps in the transversal echo canceller. For a 2B1Q signal or the like, this would require a very large number of taps because of the length of the tail involved. Unfortunately increasing the number of taps results in an increase in complexity, convergence time, and noise generated by the echo canceller.
An object of the present invention is to provide an echo canceller and echo cancellation method for echoes having relatively long tails but without involving the drawbacks mentioned hereinbefore.